1. Field of the Invention
The present invention relates to a charged particle beam extraction system and method for irradiating a charged particle beam, e.g., a proton or carbon ion beam, to a diseased part (represented by a tumor) for treatment.
2. Description of the Related Art
There is known a therapy method for irradiating a charged particle beam (ion beam), e.g., a proton or carbon ion beam, to a tumor such as a cancer in the body of a patient. A charged particle beam extraction system (ion beam extraction system) for use in such therapy comprises a charged particle beam generator, a beam transportation line, and an irradiation apparatus. An ion beam accelerated by the charged particle beam generator reaches the irradiation apparatus through a first beam transportation line and a second beam transportation line, the irradiation apparatus and the second beam transportation line being installed in a rotating gantry. The ion beam is extracted from the irradiation apparatus and irradiated to the tumor in the patient body. Known examples of the charged particle beam generator include means for circulating the charged particle beam along an orbit, means for bringing betatron oscillation of the charged particle beam into a resonant state outside the separatrix of resonance, and a synchrotron (circular accelerator) provided with an extraction deflector for extracting the charged particle beam from the orbit (see, e.g., Patent Reference 1; U.S. Pat. No. 5,363,008).
The therapy using an ion beam, in particular, the treatment with irradiation of a proton beam to a tumor, is based on characteristics that most of energy of the proton beam is released at the time when protons are stopped, namely that a Bragg peak is formed upon the stop of protons. Then, the energy of the proton beam is selected to stop protons near the tumor so that most of the energy (absorbed dose) is given only to cells of the tumor.
Usually, a tumor has a certain thickness in the direction of depth from the body surface of a patient (hereinafter referred to simply as “the direction of depth”, while it is coincident with the direction of travel of the ion beam). To effectively irradiate the ion beam over the entire thickness of the tumor in the direction of depth, the energy of the ion beam must be adjusted so as to form a comparatively wide and flat range of absorbed dose in the direction of depth (i.e., a spread-out Bragg peak width, hereinafter referred to as an “SOBP width”).
From that point of view, a range modulation wheel (hereinafter abbreviated to “RMW”) has already been proposed in which a plurality of blades each having a thickness varied step by step in the circumferential direction are disposed around a rotary shaft (see, e.g., Non-Patent Reference 1; “REVIEW OF SCIENTIFIC INSTRUMENTS”, Vol. 64, No. 8, pp. 2074-2084 and FIGS. 30-32, in particular, p. 2077 and FIG. 30 (August 1993)). In the RMW, the plurality of blades are mounted to the rotary shaft, and a through opening is formed between adjacent two of the blades. By rotating the RMW in a state where, for example, the opening is positioned on a path of the ion beam (hereinafter referred to simply as a “beam path”), the opening and the blade alternately intersect the beam path. At the time when the ion beam passes the opening, the energy of the ion beam is not attenuated and therefore the Bragg peak is produced in the deepest position inside the patient body. At the time when the ion beam passes the blade, the energy of the ion beam is attenuated at a larger rate as the ion beam passes the blade having a larger thickness, and therefore the Bragg peak is produced in a portion closer to the body surface of the patient. With the rotation of the RMW, the position in the direction of depth where the Bragg peak is formed varies cyclically. As a result, the Bragg peak width being comparatively wide and flat in the direction of depth of the tumor can be obtained, looking at the beam energy integrated over time. Further, it is known that the SOBP width can also be formed by using a ridge filter (see, e.g., Non-Patent Reference 1; in particular, p. 2078 and FIG. 31).